The present invention makes possible the manipulation of image contrast control parameters by means of a feedback process during photographing according to the user""s preference. It adjusts and controls the contrast of image data being captured in a real-time fashion to obtain any intended contrast. The invention includes the apparatus for the method.
In a conventional image system, a curve of the photosensitive characteristics represents the relation between the luminance (input brightness) of the subject and the relative reflection density in the photo. In silver halide photography, the xe2x80x9cfilm characteristics curvexe2x80x9d refers to the relationship between xe2x80x9cexposurexe2x80x9d and xe2x80x9cdensityxe2x80x9d of the film. This curve is also called D-log H curve, i.e. the characteristics curve of a typical black-and-white photosensitive material (see FIG. 1). The ratio of the variation corresponding to the transverse axis (exposure) and the longitudinal axis (density) in the characteristics curve is the slope of the curve, that is the contrast. Generally, the contrast of a photosensitive material is represented by gamma (xcex3), and the definition of the value of xcex3 is shown as the following equation:   γ  =            Δ      ⁢              xe2x80x83            ⁢      D              Δ      ⁢              xe2x80x83            ⁢      Log      ⁢              xe2x80x83            ⁢      H      
in which xcex94D represents the density difference between two measured points within the linear region of the characteristics curve, and xcex94Log H represents the log exposure difference between the two measured points.
Since the contrast characteristics of photosensitive materials generally used in the conventional imaging system, such as negative film, are predetermined by a specific development time, the photographer cannot adjust the contrast during photographing. To accommodate the lighting conditions at the scene, the contrast of the negative film is adjusted by changing its development time. The world famous Zone System is developed for this purpose. Further, U.S. Pat. No. 5,445,929 discloses a chemical-based variable contrast photographing system, which lets the user adjust the contrast effect of a film. However, this method shows the effect only after the film has been chemically processed; it is unable to show the contrast effect at the site or to allow the user to change the contrast during photographing.
As can be seen, the contrast characteristics of regular film are predetermined, and the dynamic range of the recordable scene light is therefore fixed. When capturing the image of a scene, the photographer can only adjust the exposure with the diaphragm opening and the shutter to obtain image input signals of a certain range of light for specific exposure latitude. U.S. Pat. No. 5,159,384 discloses a precise and complicated exposure system which improves the quality of the image input signals. However, it cannot change the values of the characteristics curve of the image system itself.
Conventionally, scene contrast is further directly associated with the subject""s luminance range or the relative brightness ratio of the objects under existing lighting condition at the scene. According to past studies, the relative brightness ratio of the most general scene being photographed is within about 20:1 to 800:1, and more particularly 160:1 as a common value. Therefore, this scene contrast is deemed to be a standard contrast for the manufacturer for determining the contrast characteristics of regular negative film. Thus the dynamic range of the recordable scene light of negative film is about 7xc2xc diaphragm stops. FIG. 2 is an example of an ideal negative film with a dynamic range of scene light at 100:1, the X-axis of its characteristics curve representing the log H (domain of input signal) of the relative brightness ratio between different objects in the scene, and the Y-axis of the characteristics curve representing the density value (value of output signal) developed from the film. If the reference exposure value of a camera is designated to the center of the desired range of the input signals, the characteristics curve of the film will shift horizontally along X-axis while the reference exposure value is being adjusted (essentially, changing the exposure setting on a camera). However, the slope of the curve remains unchanged.
Therefore, as shown in FIG. 2, if the reference exposure value is set in the middle of curve A, the objects within the brightness range of E0-E2 will be recorded on the film as density within 0-2, and the objects of any brightness range above E2 will be overexposed and not distinguishable. When the reference exposure value of the camera is set in the middle of curve B, the objects within the brightness range of E1-E3 will be recorded on the film as density within 0-2, and the objects of any brightness range below E1 or above E3 will be underexposed or overexposed, respectively. Similarly, when the reference exposure value of the camera is set in the middle of curve C, the film can only record objects of a brightness range within E2-E4. Consequently, the slope of the contrast curve of the film remains unchanged when the user changes the reference exposure value (exposure setting) of the camera, and the film can only record an object within a specific dynamic range of the input signals. In addition, when the input dynamic range is being changed without affecting its output signal value (the density range of the film after development), the function representing the contrast characteristics of the photographing system must be changed as well. For example, when the slope of curve B in FIG. 3 is increased, the dynamic range of input signals will hence be reduced toward curve A. If the slope of curve B is reduced, the dynamic range of input signals will be increased toward curve C. However, the range of output signals and the reference exposure value of the camera remain unchanged for all these curves.
Therefore in silver halide photography the characteristic curves of the film defines the dynamic range of input signals, i.e. the recordable range of scene brightness. When the user wants to adjust the recordable dynamic range of a scene, it is necessary to change the characteristics curve of the photographic system. Referring to curve A in FIG. 4, if X1, X2, Y1, Y2 are known, the slope of curve A shows a linear characteristic represented by the function Y=mX+c, in which m is the slope, c is the constant. If the value X3 is added, the curve can be shaped up with X3xe2x80x2 to become curve B as a non-linear characteristic function as shown in FIG. 4. In practice therefore, if XN is the brightness value of a certain object in the scene selected by the photographer, and YN is the value of the output signals defined by the photographer, the slope m of the characteristics curve can be calculated through the following equation in accordance with at least two sets of selected values (XN, YN):   m  =                    Y        2            -              Y        1                            X        2            -              X        1            
By means of adjusting the value m, the photographer can adjust the contrast value of the system upward or downward. In some cases it is not necessary to use up the full range of its output value in order to set the value Y1-Y2 needed for the user""s preference. Although U.S. Pat. No. 5,539,459 has disclosed a similar concept, it neither provides the functionality for a user to control and adjust systems contrast value, nor describes how the system controls and adjusts its contrast value subject to the user""s instructions.
As shown in FIG. 5, in the prior digital camera 1, a CCD (charges couple device) 11 is used to capture the scene image through a lens 10 and to convert the image being captured into analog signals. These analog signals are then converted into digital image signals through an analog-to-digital converter 12 and then encoded by an encoder 13, stored in a memory (or storage device) 14 and output through a display 15.Thus the control parameters 16 were set statically with predetermined criteria by controlling its gain and offset to adjust the system characteristics of a linear function similar to Y=mX+c, as disclosed in U.S. Pat. No. 4,187,519. The static principle underlying the patent does not include any concept of building up a non-linear function relationship, and does not allow the value of the input signal for use in the contrast-adjusting process to be set by the user. Since visual contrast in human visual systems varies with the intensity of the scene light, a direct mapping from the dynamic range of input scene light to the whole range of output signals will not satisfy the photographer""s needs for controlling and adjusting the characteristics of image contrast in real-time according to his preference.
The present invention provides an image processing method by way of manipulating the image contrast control parameters through a feedback process, and it defines the related apparatus. It calculates a new contrast control parameter subject to the projection of the scene light measured and the control parameter input by the user, and it stores this new parameter in a control parameter memory. The parameter can now be used to adjust the contrast of the image data being duly captured, and to display the image data being adjusted through a display (for example, LCD) of the user interface, enabling the user to freely adjust the contrast control parameters through a feedback process subject to his preference. Thus the apparatus is capable of calculating a linear or non-linear contrast characteristics curve subject to human visual characteristics (for example, Steven""s effect), and for using it as a sampling reference value. Using the apparatus, for example, as a digital camera or a digital video camera, to capture the image of a scene, it is now possible to obtain the exact contrast value desired.